Internal combustion engines combust mixtures of air and fuel to generate mechanical power for work. The basic components of an internal combustion engine are well known in the art and preferably include an engine block, cylinder head, cylinders, pistons, valves, crankshaft and one or more camshafts. The cylinder heads, cylinders and tops of the pistons typically form variable volume combustion chambers into which fuel and air are introduced and combustion occurs as part of a thermodynamic cycle of the device. In all internal combustion engines, useful work is generated from the hot, gaseous products of combustion acting directly on moveable engine components, such as the top or crown of a piston. Generally, reciprocating motion of the pistons is transferred to rotary motion of a crankshaft via connecting rods. One known internal combustion engine operates in a four-stroke combustion cycle, wherein a stroke is defined as a complete movement of a piston from a top-dead-center (TDC) position to a bottom-dead-center (BDC) position or vice versa, and the strokes include intake, compression, power and exhaust. Accordingly, a four-stroke engine is defined herein to be an engine that requires four complete strokes of a piston for every power stroke of a cylinder charge, i.e., for every stroke that delivers power to a crankshaft.
The overall efficiency of an internal combustion engine is dependent on its ability to maximize the efficiency of all the processes by minimizing the compromises that lead to energy losses to the environment. Dividing the traditional four-stroke cycle amongst dedicated components allows the compression process to be made more efficient by attempting to approximate isothermal compression of a cylinder charge through mid-compression heat extraction, such as by using a heat exchanger. Likewise, a greater amount of energy may be harnessed during expansion of a cylinder charge by moving towards an adiabatic expansion, and extending that expansion further to bring the working gases down to atmospheric pressure. In addition, maximizing the ratio of specific heats of the working gas while reducing each specific heat individually allows greater energy extraction over the expansion while minimizing the mechanical and flow losses associated with each dedicated component.
One known approach to meeting these challenges is a low temperature combustion (LTC) turbocharged diesel engine. The LTC turbocharged diesel relies on a two-stage compression process separated by charge cooling to approximate isothermal compression, reducing the work required to achieve a given air density, lean low temperature combustion to minimize heat losses while improving gas properties, and a two-stage expansion process to enhance work recovery from the hot post-combustion gases. Thermodynamically, the turbocharged diesel is a multi-shaft dual-compression, dual expansion engine that relies on a combination of rotating and reciprocating machines to execute two compressions prior to combustion and two expansions post-combustion. However, the overall efficiency may be limited by the ability to match and optimize the performance of these components over the operating domain. Air handling systems used to provide boosting on externally-charged multi-shaft engines may include more complex boosting systems using two and three stages of turbocharging or combinations of turbochargers and mechanically driven superchargers. In addition to the charging devices, the systems require heat exchangers, bypass valves and controls.